Production printing systems for high-volume printing typically utilize a production printer that marks a continuous-form print medium (e.g., a web of paper) with a wet colorant (e.g., an aqueous ink). After marking the print medium, a dryer downstream from the production printer is used to dry the colorant. One such dryer is a microwave dryer that uses microwave energy to heat the colorant to cause a liquid portion of the colorant to evaporate, thereby fixing the colorant to the print medium. The primary mechanism of heating the colorant is dielectric heating.
In a typical microwave dryer, a microwave source directs the microwave energy down a long axis of a waveguide which spans across the width of the print medium. The print medium travels through a short axis of the waveguide via a passageway through the waveguide. As the continuous-form print medium traverses the short axis of the waveguide, the wet colorants applied to the continuous-form print medium are exposed to the microwave energy and dried.
One problem with such microwave dryers is that the microwaves emitted by the microwave source are standing waves inside the cavity of the waveguide. That is, the electromagnetic field along the long axis of the waveguide oscillates in intensity with high power density at peaks of the wave and low power density between the peaks of the wave. Variation of the field intensity across the width of the web leads to heating variations across the width of the web and non-uniform drying of the print media.